Devices of this general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Often, such injection devices are required to work with glass pre-filled syringes that were originally designed for manual use. Such glass syringes have a flange at their base to allow a user to grip the syringe. The substantial force produced by the drive spring is applied to the piston of the syringe. This force is transferred to the housing and return spring, via syringe carrier. The syringe carrier is normally sheath which is designed to envelop the syringe and take up forces applied to the syringe to prevent damage to the frangible glass body of the syringe.
The syringe is manufactured with a boot which covers its needle. The aim of the boot is to protect the needle and maintain its sterility. The needle is joined to the glass body of the syringe by an integrity seal. With injection devices of the present invention, the syringe boot may be connected to the syringe body via a frangible connection, or, alternatively, the boot may be a tight rubber boot covering the needle. In either case, the boot is gripped by a cap of the injection device so that the boot becomes removed when the cap of the injection device is removed prior to use.
In current injection devices, the syringe carrier is nominally biased into the syringe by a return spring. The bias is only overcome when a drive spring is released which forces the syringe carrier against the bias of the return spring to move the syringe into an extended position whilst its contents is ejected. However, before actuation of the drive spring, the syringe carrier is still free to move against the return spring when high loading forces are applied externally to the injection device, for example during impact of the injection device with a hard surface, such as when the device is dropped. In such situations, since the boot is held rigidly in the cap of the injection device, movement of the syringe carrier (and syringe) may disturb the integrity of the needle seal with the syringe or cause the frangible connection between the boot and the syringe to break. Of course, this exposes the needle and its contents to a non-sterile environment which is undesirable.